In Japan, which is an earthquake-prone country, a variety of quake resistant techniques, seismic isolation techniques, and vibration control techniques, such as techniques against seismic force and techniques for reducing seismic force on buildings, have been developed for a variety of constructions, such as buildings, bridges, elevated roads, and single-family houses, and have been applied to a variety of constructions.
In particular, seismic isolation techniques, which are the techniques for reducing seismic force on constructions, can effectively reduce vibrations in constructions when earthquakes occur. According to the seismic isolation techniques, a seismic isolation device is provided between a base, which is a lower structure, and an upper structure so that transmission of vibration of the base, which occurs due to an earthquake, to the upper structure is reduced and vibration of the upper structure is thus reduced. Thus, the structure stability is ensured. Such a seismic isolation device is effective not only when an earthquake occurs but also for reducing the influence of traffic vibration, which always acts upon the construction, on the upper structure.
Examples of seismic isolation devices include devices with a variety of configurations, such as a lead plug-containing laminated rubber bearing device, a high damping laminated rubber bearing device, a device that combines a laminated rubber bearing and a damper, and a sliding seismic isolation device. Above all, the sliding seismic isolation device will be exemplarily described with reference to its general structure. A sliding seismic isolation device typically includes upper and lower shoes each having a sliding surface with a curvature, a columnar slider interposed between the upper and lower shoes and having upper and lower surfaces that are in contact with and have the same curvatures as the upper and lower shoes, respectively. Such a sliding seismic isolation device is also referred to as a seismic isolation device with slidable upper and lower spherical surfaces or a double-concave seismic isolation device.
In this type of seismic isolation device, the operation performance of the upper and lower shoes is dominated by the coefficient of friction between the upper and lower shoes and the slider interposed between them or by frictional force that corresponds to the coefficient of friction multiplied by the weight.
By the way, in the conventional sliding seismic isolation device, the reference contact pressure of a slider is less than or equal to 20 MPa. Therefore, when the weight of a construction is increased by an increase in the height thereof or the like, there is no way other than increasing the size of the sliding seismic isolation device correspondingly so that the device has planar dimensions that can withstand the load of the construction. This results in lower cost competitiveness of the device in comparison with other types of seismic isolation devices, such as laminated rubber seismic isolation devices. Thus, such a sliding seismic isolation device has come to be used less frequently.
It should be noted that when a slider formed of steel is applied, the slider can be machined with high precision as it is mechanically machined. However, as there is a large variation in the coefficient of friction, and as the contact pressure dependence and the velocity dependence of the coefficient of friction are high, the range between the upper and lower limits of the coefficient of friction is large. Thus, the earthquake response is likely to vary, which is problematic. Therefore, even if such a slider can prevent collapse of a building, the slider is difficult to be a constituent member of a high-performance seismic isolation device that can have PML (probable maximum loss) that is close to zero and can cause no damage to furniture and fixtures and the like.
Patent Literature 1 discloses a sliding seismic isolation device including substrates, each of which has a laminated body of fiber woven fabric-reinforced thermosetting synthetic resin, and a slider having surface layer materials that are integrally joined to the upper and lower surfaces of the respective substrates.
Such a slider is formed by superposing plain-woven PTFE fibers or by superposing plain-woven PTFE fibers, a woven fabric, and a plain-woven cotton cloth. A slider with such a structure is expected to have reduced frictional properties derived from PTFE.
However, as is clearly shown in the experimental conditions disclosed in Patent Literature 1, the contact pressure of the slider disclosed in Patent Literature 1 is also 19.6 N/mm2 (19.6 MPa), which is less than 20 MPa. Thus, it would be impossible to solve the problem resulting from the low contact pressure of the slider described above.